1. Field of the Invention
The present invention relates in general to drive trains for automotive vehicles and more particularly to a four-wheel drive system equipped with a viscous coupling.
2. Description of the Prior Art
An example of a prior art four-wheel drive system is adapted to transmit a driving force from an engine to a set of front wheels through a mechanically engaged drive train therebetween and to a set of rear wheels by way of a viscous coupling in such a way that the driving force is distributed to the front and rear wheels based on the difference in rotation between the input and output shafts of the viscous coupling for thereby making the distribution of the driving force comply with the road surface and driving conditions.
The prior art four-wheel drive system is constructed so that the speeds N i, N o of rotation of the input and output shafts of the viscous coupling are equal to each other when the speeds N f, N r of rotation of the front and rear wheels are equal to each other and that the difference .DELTA.N io (=N i-N o) in speed of rotation between the input and output shafts of the viscous coupling is caused in response to the difference .DELTA.N fr (=N f-N r) in speed of rotation between the front and rear wheels. The drive system thus effects a two-wheel drive mode of operation unless there is any difference .DELTA.N fr in speed of rotation between the front and rear wheels, resulting in a tendency that a driving force distributed to the rear wheels is smaller than that to the front wheels.
When the drive system is in a two-wheel drive mode of operation, a relation of N f&gt;N r exists since the front wheels slips a small amount. As a result, a driving force proportional to the difference .DELTA.N fr in speed of rotation between the front and rear wheels as represented by the dotted lines in FIG. 3 is distributed to the rear wheels. However, since the difference .DELTA.N fr is so small, a driving force F f distributed to the front wheels is far larger than that F r to the rear wheels except some particular moments.
Due to this, the prior art drive system effects a responsive delay when responding to a positive difference .DELTA.N fr in speed of rotation between the front and rear wheels (the positive difference arises when the speed of rotation of the front wheels which are always driven is higher than that of the rear wheels) for changing the driving mode of operation from the two-wheel drive mode to the four-wheel drive mode and thereby improving the acceleration efficiency at starting, the straight running stability and driveability on a slippery road.
Further, at the time of braking which makes the difference in speed of rotation between the front and rear wheels, .DELTA.N fr is negative and the front wheels tend to lock, the driving force is transmitted to the front wheels to improve braking efficiency. However, at this time, a high braking torque is transmitted to the rear wheels by an action of the drive system reverse to that effected when there is a positive difference .DELTA.N fr in speed of rotation between the front and rear wheels. Due to this, the rear wheels tend to lock on engine braking during running on a slippery descending road and with foot braking, resulting in a poor, unreliable braking stability.